Innerspring assemblies for mattresses or seating structures such as sofas are generally composed of a plurality of spring coils tied together in a matrix or array. In a mattress innerspring, border wires usually encircle both the upper and lower perimeters of support surfaces formed by the ends of the vertically oriented coils. The border wires are connected to convolutions of the perimetrical springs by hog rings. The border wires are attached to top and bottom convolutions of the perimeter coils. Alternatively, the coils may have offset sections located near the ends which extend radially beyond the terminal convolutions, and wherein the terminal convolutions extend axially beyond the offset sections, as described for example in Sealy U.S. Pat. No. 5,713,088. It is a common practice to overlap the terminal convolutions of adjacent spring coils in a row, and then wind even smaller diameter helical spring coils, referred to as cross-helicals, across the rows to encircle the overlapped terminal convolution portions. Such an innerspring construction is described for example in Sealy U.S. Pat. No. 4,726,572. Other types of innersprings may have spring and support elements made or plastic or composite materials.
With respect to the perimeter of the innerspring assemblies of mattresses, there are some general considerations of construction and manufacture. In the normal use of a mattress, the edges of the innerspring are subjected to greater compression forces than the interior of the innerspring, largely due to the common practice of sitting on the edge of the bed. The added stresses and strains on the edges of the innerspring are evident in a general rounding of the mattress at the perimeter, creating a condition known as "roll-off". This is especially true of the edges and corners of a mattress which are the weakest structural points of the mattress, particularly in lower end mattress designs in which the innerspring assemblies rely entirely on the weight bearing ability of the coils for the mattress to maintain its shape. The innerspring can further give the impression of a degree of softness it does not have, since a person sitting on the edge provides a much more concentrated load on the underlying springs than a prone body upon a central portion of the innerspring.
Different types of reinforcements have been used in connection with innerspring assemblies to overcome these weaknesses. For example, Sealy U.S. Pat. No. 5,787,532 describes various foam structures interlockingly engaged with mattress innersprings to improve the shape and support characteristics. Some of the foam shapes described, though highly functional, are rather complex and therefore somewhat difficult to manufacture and assemble. Extrusion of foam pieces of different cross-sectional configurations has been proven to be efficient, so long as the shape is not overly complex. Also, dimensional tolerance is sometimes difficult to achieve given the somewhat unpredictable expansion behavior of different types of foam as it exits the extrusion die. Therefore, foam pieces of relatively simple cross-sectional configuration are preferable for consistency. Also, smaller and more compact foam pieces are easier to engage with innerspring assemblies. Large or long structures have a tendency to disengage before the padding and upholstery is attached over the foam to secure it in place. The assembly of mattresses is largely a manual process. In particular, the installation of padding and upholstery to the top and bottom surfaces of the innerspring and about the borderwire is very difficult. Much dimensional variation can occur in the process, due to deflection of the springs and compression of the padding material, resulting in uneven seam lines. Adding structural elements to the innerspring.